The power supply for a backlight source of a TFT LCD panel makes use of an inverter circuit to accomplish energy conversion and turn a cold cathode fluorescent lamp (CCFL) on. Conventional inverter circuits can be divided into half bridge-type, full-bridge type and push/pull-type according to different circuit topologies. An inverter circuit is a circuit for converting DC power into AC power.
As shown in FIG. 1, a transformer T1 divides the circuit into a front-end circuit at the primary side 101 and a rear-end circuit at the secondary side 102. The front-end circuit at the primary side 101 comprises a DC voltage source Vcc, a first switch Q1, and a second switch Q2. The rear-end circuit at the secondary side 102 comprises at least a capacitor (C1, C2, C3), a load, and at least a diode (D1, D2). A push/pull-type control chip 103 is connected between the front-end circuit at the primary side 101 and the rear-end circuit at the secondary side 102. Reference is also made to FIG. 2. The push/pull-type control chip 103 outputs a first control signal a and a second control signal b to turn switching actions of the two switches Q1 and Q2 at the primary side 101, respectively. The DC power source Vcc is used to provide energy, and the transformer T1 raises and converts the voltage of the DC power Vcc to the rear-end circuit 102 for driving the load. The output voltage waveform c at the secondary side of the transformer T1 is the voltage waveform at point C. As shown in FIG. 2, the output voltage waveform c at the secondary side is an AC voltage waveform.
In the above description, the push/pull-type control chip 103 can be the LX1686, LX1688 or LX1691 push/pull-type control chip produced by Linfinity (Microsemi) Corporation, the 02-9RR push/pull-type control chip produced by O2Micro International Limited, or the BIT3494 push/pull-type control chip produced by Beyond Innovation Technology.
As shown in FIG. 3, a transformer T2 divides the circuit into a front-end circuit at the primary side 201 and a rear-end circuit at the secondary side 202. The front-end circuit at the primary side 201 comprises a DC voltage source Vcc, two electronic switches (Q1, Q2), a half bridge-type control chip TL494, two capacitors (C1, C2) and an isolation transformer Tr. The rear-end circuit at the secondary side 202 comprises a load. Reference is also made to FIG. 4. The half-bridge control chip TL494 outputs control signals D1–D2 via two output terminals D1 and D2. The control signals D1–D2 control switching actions of the two electronic switches Q1 and Q2 via the isolation transformer Tr, respectively. The two electronic switches Q1 and Q2 are n-channel FETs or p-channel FETs. Through switching actions of the two electronic switches Q1 and Q2, electric energy stored in the capacitors C1 and C2 can be transferred to a primary side terminal T21 of the transformer T2 via a coupling capacitor C3 to form an AC power source ac. The voltage of the capacitors C1 and C2 is a half (Vcc/2) of the DC voltage Vcc. The AC power source ac is used to provide energy for the transformer T2, which boosts and converts the AC power source to the secondary side 202 for driving the load.
In the above description, if the inverter circuit used is of the half bridge-type, a half bridge-type control chip needs to be matched for normal operations, while if the used inverter circuit is of the push/pull-type, a push/pull-type control chip needs to be matched for normal operations, hence having less flexibility and commonality in practical use. In other words, control chips can't be jointly used and purchased together, or a more complicated circuit needs to be matched.
Further, for a conventional half bridge-type inverter circuit, an isolation transformer is required to control switching actions of two electronic switches. The half bridge-type control chip can't directly drive the two electronic switches. Moreover, both the two electronic switches used in a conventional half bridge-type inverter circuit are n-channel FETs or p-channel FETs.